1. Technical Field
The present invention relates to a nanoparticle and a method of producing the nanoparticle, and in particular, relates to a core/shell particle and a method of producing the same.
2. Description of the Related Art
Techniques of coating a material for a core with a material for a shell in order to protect a chemically or physically unstable material, or give special functions thereto, are known. Particles manufactured by such techniques are generally called core/shell particles.
Many applications of core/shell particles are known, such as silver halide particles for photographic photosensitive materials and silicone elastomer particles in a thermosetting resin. In recent years, core/shell particles as applied to magnetic particles, especially magnetic particles in magnetic recording media, have attracted significant attention.
Miniaturization and advances in performance of magnetic recording media for computers such as magnetic tapes and magnetic discs have been progressing due to densification of recording capacity. Accompanying this, reduction in particle size of magnetic substances has also been progressing. Among ferromagnetic materials having the same mass, ones having a smaller particle size can achieve lower noise.
For example, CuAu type and/or Cu3Au type ferromagnetic ordered alloys are promising materials for improving magnetic recording density because they have great crystal magnetic anisotropy due to distortion that occurs in ordering, and thus exhibit ferromagnetism even when their particle size is reduced to a state that is generally referred to as a nanoparticle.
Metal nanoparticles can be produced by a liquid phase method. As the liquid phase method, various methods which have been conventionally known can be employed. Examples of the liquid phase method include, according to classification by the precipitation method, (1) alcohol reduction methods in which a primary alcohol is used, (2) polyol reduction methods in which a secondary, tertiary, dihydric or trihydric alcohol is used, (3) thermal decomposition methods, (4) ultrasonic decomposition methods, (5) reduction methods with a potent reducing agent, and the like. Furthermore, according to classification based on reaction systems, examples of the liquid phase method include (6) polymer existence methods, (7) high-boiling point solvent methods, (8) regular micelle methods, (9) reverse micelle methods and the like. As the liquid phase method, a reduction method established by improvement of these methods is preferably employed, and among the reduction methods, the reverse micelle method which is capable of readily controlling the particle diameter is particularly preferred.
The metal nanoparticles produced by the liquid phase method may be subjected to the annealing treatment as necessary. For example, in the case of Cu/Au type or Cu3Au type ferromagnetic ordered alloy, the alloy nanoparticle synthesized according to the aforementioned method has a face centered cubic crystal structure. The face centered cubic crystal usually exhibits soft magnetism or paramagnetism, and those exhibiting soft magnetism or paramagnetism are not suited for recording media. In order to obtain a ferromagnetic ordered alloy having a coercive force of 95.5 kA/m (1200 Oe) or greater, which is required for magnetic recording media, an annealing treatment must be carried out at a temperature equal to or higher than the transformation temperature at which the alloy is transformed from its disordered phase to the ordered phase. However, when the alloy nanoparticles produced according to the above method are applied on a support and subjected to the annealing treatment to produce a magnetic recording medium, the alloy nanoparticles tend to flocculate easily with each other leading to reduced coatability and deteriorated magnetic properties. In addition, due to uneven particle diameter of the resulting alloy nanoparticles, it has been difficult to form a perfect ordered phase even if a heat treatment is executed. Accordingly, there have been cases in which the desired ferromagnetism is not achieved.
As methods of preventing fusion of the metal nanoparticles resulting from annealing, those described in Japanese Patent Application Laid-Open (JP-A) Nos. 2003-132519 and 2003-217108, and Japanese National Phase Publication No. 2003-533363 and the like are known. However, according to these methods, condensation of the metal alkoxide compound used as the shell may proceed excessively, which may cause flocculation of the core/shell particles and generation of particles of the metal alkoxide compound alone as a byproduct.